[flang] [llvm] [flang] implement `split` for fortran 2023 (PR #161484)

[Tom Eccles via llvm-commits]

================
@@ -0,0 +1,43 @@
+! RUN: bbc -emit-fir -hlfir=false %s -o - | FileCheck %s
----------------
tblah wrote:

In those cases back is statically present or absent (known at compile time). You can make it dynamic (aka only known at runtime) by using a subroutine argument which is itself optional.

https://github.com/llvm/llvm-project/pull/161484